1. Field of the Invention
The present invention relates to a charger for use in image forming apparatuses such as printers, video printers, facsimile machines, copying machines, and displays. More particularly, the present invention concerns a charger for effecting a charging process or a discharging process by causing a charging member with a voltage applied thereto from the outside to be brought into contact with a member to be charged.
2. Description of the Background Art
Hereafter, a description will be given of a charger used for an image forming apparatus for image formation by means of electrophotography using a photosensitive member as a member to be charged.
A charger is known which effects a charging process by causing a charging member with a voltage applied thereto from the outside to be brought into contact with a member to be charged. In this charger, the charging member is brought into contact with the surface of a photosensitive member, i.e., the member to be charged, which produces a gap (i.e., a discharging gap) in the vicinity of a contact portion between the charging member and the photosensitive member, and the photosensitive member is charged through a discharging phenomenon occurring at this gap. This charger has come to attract attention and has been commercialized since it has advantages in that, as compared with a corona charger, a low voltage power supply can be used and the amount of ozone produced is very small.
Conventional charging members include an electrically conductive fiber brush such as the one disclosed in Japanese Patent Application Laid-Open No. 19837/1980, an electrically conductive resilient roller such as the one disclosed in Japanese Patent Application Laid-Open No. 132356/1981, and an electrically conductive blade such as the one disclosed in Japanese Patent Publication No. 14701/1990.
Furthermore, in recent years, chargers which use flexible films as charging members have been proposed.
Japanese Patent Application Laid-Open No. 86681/1992 discloses a charge which is arranged such that both ends of a flexible film (which is stated as a sheet in its specification) are supported, and the center of a slack portion is brought into contact with a photosensitive member. In addition, U.S. Pat. No. 5,192,974 discloses a charger which is arranged such that one end of a flexible film is supported, and a free end thereof is brought into contact with a photosensitive member. Furthermore, U.S. Pat. No. 5,243,387 discloses a charger which is arranged such that a tube having an inside diameter greater than the diameter of a rotatable roller is fitted over the roller, and a side of the tube located away from a photosensitive member is pressed against the roller, thereby causing a slack portion to be brought into contact with a photosensitive member.
In addition, to secure a uniform charging property, applications for patent have been filed which specify the surface roughness of charging members. For example, Japanese Patent Application Laid-Open No. 132356/1981 discloses the relationship between the surface roughness of an electrically conductive roller and nonuniformity in charging. U.S. Pat. No. 5,008,796 specifies the relationship in surface roughness between a charging member and a photosensitive member. Japanese Patent Application Laid-Open No. 198468/1990 specifies a range of the maximum roughness of a charging member.
However, in the case of the charger disclosed in Japanese Patent Application Laid-Open No. 86681/1992, the state of contact between the charging member and the photosensitive member cannot be determined precisely since it depends upon the manner in which the film is slackened. Since the state of contact is indeterminate, the discharging gap formed in the vicinity of the contact portion is also indeterminate and is unstable. Accordingly, there exists a problem in that uniformity in charging cannot be obtained. Furthermore, since the state of contact of the film is indeterminate, there exists another problem in that the film taps the photosensitive member, generating charging noise.
In the case of the charger disclosed in U.S. Pat. No. 5,243,387 as well, the state of contact between the charging member and the photosensitive member is indeterminate since it depends upon the manner in which the tube is slackened. Accordingly, there exists the problem that uniformity in charging cannot be obtained. Additionally, there exists a problem in that the arrangement is complex.
Furthermore, in the case of the charger disclosed in U.S. Pat. No. 5,192,974, if a ridgeline of the free end of the film is slightly deformed, or if the ridgeline is formed inaccurately, the contact between the film and the photosensitive member becomes nonuniform. As a result, the discharging gap formed in the vicinity of the contact portion becomes nonuniform and lacks stability. Hence, there exists the problem that uniformity in charging cannot be obtained.
It is mentioned in Japanese Patent Application Laid-Open No. 86681/1992 that an electric field is produced between the charging member and a rear surface-side electrically conductive layer of the member to be charged, and an electric force is generated. In this publication, however, that electric force is only ascertained as a cause for the generation of vibration or noise at the time of superposition of an ac voltage, and there is no consideration given to actively using the electric force as a pressure contact force.
Accordingly, the present invention has been devised to overcome these problems, and its object is to provide a charger capable of positively and uniformly maintaining a discharging gap formed in the vicinity of a contact portion between the charging member and the photosensitive member.
Another object is to provide a charger which avoids the frictional deterioration of the photosensitive member or the charging member and is capable of effecting a stable and reliable charging process.
Still another object is to provide a charger which prevents foreign objects, such as toner, toner additives, paper dust, and the like, from to remaining in the vicinity of the contact portion between the charging member and the photosensitive member.